1. Field of the Invention
The present invention relates to an apparatus for producing high-purity water to be used in the semiconductor manufacturing industry or the like.
2. Description of the Related Art
In general, high-purity water (in the specification, the term “high-purity water” refers to highly pure water such as “pure water” and “ultra-pure water” for which no general definition exists) is produced by treating raw water such as river water, ground water, and industrial water through a pretreatment process to remove most of suspended and organic substances from the raw water and subsequently treating the pretreated water by a primary pure water producing device and a secondary pure water producing device (also referred to in the specification as a “subsystem”). The produced high-purity water is then supplied to a point-of-use such for example as wafer washing in the semiconductor manufacturing industry. The high-purity water is highly pure to such a degree that even a quantitative analysis of the impurities is difficult, but still contains some impurities. The influences of these minute amount of impurity compositions contained in the high-purity water on products such as semiconductor devices cannot be ignored in devices of high degree of integration (highly integrated devices), and, therefore, there is a strong demand for obtaining high-purity water having an even higher purity than the conventional high-purity water.
For example, the high-purity water produced in the subsystem is supplied to the point-of-use through pipes, but the pipes between the subsystem and the point-of-use in some cases are several hundreds of meters long in length. Because of this, impurities such as particulates (or particles) and metal ion compositions mix from the pipes into the high-purity water in a very small amount and sometimes adversely affects characteristics of devices. For example, metal pollution may adversely affect the electrical characteristics of the devices and particles may cause pattern defects, disconnection, and defects in dielectric strength. In addition, the compositions that are not removed in the high-purity water producing apparatus may also adversely affect characteristics of the devices, and when, for some reason, there is a leak from the high-purity water producing apparatus instantaneously or for a short period, the device characteristics may be adversely affected.
As a solution for such contamination, methods for further treating the high-purity water immediately before the point-of-use have been proposed. For example, Japanese Patent Laid-Open Publication No. Hei 4-78483 discloses a method for providing a column filled with ion exchange fiber immediately before the point-of-use. Japanese Patent Laid-Open Publication No. Sho 58-81483 discloses a method for providing an ultrafiltration membrane module immediately before the point-of-use. Each of these methods, however, has a disadvantage that because the primary target of the column filled with ion exchange fiber is ionic impurities, the removal performance for the particulates is not sufficient, and although the ultrafiltration membrane module can remove particulates, it cannot remove dissoluble impurities such as ions.
In addition, an ion adsorption module which is filled with an ion adsorption membrane is known as a module having both particulate and ionic impurity removal functions. Known configuration of the ion adsorption membrane to be used in such a module include a plate type and a spiral type which are created from a planar membrane, a tubular type, and a hollow fiber membrane (Japanese Patent Laid-Open Publication No. Hei 8-89954). Suitable membrane configurations are determined and commercialized based on the objective of the use, etc. However, because these ion adsorption membranes are formed by introducing an ion exchange group or the like to an olefin-based porous membrane or the like through radical polymerization or radiation-induced polymerization, there are problems in that the introduction density of the ion exchange group or the like to be introduced is low and in that the section to be introduced is limited to a region proximate to the membrane surface. In other words, because the introduction density of the ion exchange group or the like is low, the ion adsorption capacity of the module as a whole is small and because the ion exchange group or the like is not uniformly distributed, the adsorbed ions tend to spread within the module in the direction of the flow, causing a longer ion exchange band which is a mixed region of the ion adsorbing section and non-adsorbing section within the module and greater tendency for a small amount of leakage to occur. Because of this, when the module is used as a module for the final treatment of high-purity water wherein impurities in the order of nanograms per liter (ng/L) or picograms per liter (pg/L) may be inimical, the purity of the treated water leaves much to be desired, and thus, there is disadvantage in that the module must be frequently replaced.